How can two solutions have different ORP values and yet the same pH value?
i.e. isn't pH the measure of the OH-'s, i.e. hydroxyls, in a solution, and isn't this the measure of the extra/free electrons available [for redox], i.e. the ORP?
1 Answer | Add Yours
Redox potential value depends upon the willingness or ease with which some species accepts (or releases) electrons, while pH is negative logarithm of H+ (or OH-) ion concentration in solution. For systems that does not involve H+ (or OH-) ions, the redox potential of it does not depend upon pH. e.g. Fe3+/Fe2+ has a potential of 0.77V, independent of pH. But for redox systms that involve transfer or dischrge of H+ or OH- ions, redox potential is pH dependent. O2 + 2H2O + 4e- gives you 4OH- has a redox potential of 0.40V at 1M conc, at other concentrations, the potential value changes according to the Nernst equation: E = E0 - .059/n log [red]/[ox]. So for systems where OH- is the only species undergoing redox change, your argument stands good, and definitely concentration of OH- governs pH and also governs the ORP value, albeit through a different set of mathematical operator. For other systems exact relationship between H+ (or OH-) ions and the actual redox process has to be settled first and then an equation interrelating pH and ORP can be arrived at.
We’ve answered 319,859 questions. We can answer yours, too.Ask a question